1. Field of the Invention
The present invention relates to means and method for spray-forming a metal, polymer, or metal/polymer matrix deposit on a substrate and, in particular, to means and method for pressurized injection liquid-feed spraying.
2. Discussion of the Prior Art
This invention relates to a method and apparatus for producing a spray of finely atomized liquid droplets of controlled size distribution, velocity, heat content, flux, and flow pattern. The primary function of the device is to spray form near-net-shape solids and coatings of metals, polymers, and composite materials by directing a spray of atomized droplets onto a suitably shaped substrate or mold. Powders of these materials are produced by allowing the droplets to solidify in-flight.
Tremendous growth in the science and technology of atomization has occurred in the past decade. The discipline is now recognized as a major international field of research. Atomization of liquids involves the disintegration of a bulk liquid into fine droplets, and devices used to generate atomized sprays are designated as atomizers or nozzles. Methodologies for generating sprays include discharging a liquid at high velocity into a relatively slow-moving stream of air or other gas, the ejection of a liquid from the periphery of a cup or disk rotating at high velocity, and the exposure of a relatively slow-moving liquid to a high-velocity gas. The latter approach is employed in the present invention.
Atomized sprays find use in a wide range of applications including spray-drying, cooling, combustion, painting, and powdered metal production. Spray-forming is another application of atomized sprays but differs in that atomized droplets of engineered alloys, plastics, and composite materials are spray-deposited onto a suitably shaped substrate or pattern to produce a free-standing, near-net-shape, or net-shape solid. The properties of the spray-formed product reflect the interplay of the characteristics of the spray plume and substrate onto which the spray is deposited. Spray-forming can offer unique opportunities for simplifying materials processing without sacrificing and, oftentimes substantially improving, product quality. In addition to near-net-shape fabrication capabilities, spray-forming is applicable to a wide range of metals and nonmetals and offers property improvements through rapid solidification (e.g., in the case of metals, refined microstructures, extended solid solubilities, and reduced segregation). Economic benefits result from process simplification and the elimination of unit operations. In addition to general spray-forming applications, the present invention has also been used to form coatings and powders of metals, polymers, and composite materials.
This instant invention is an improvement to the spray-forming process which has been developed at the Idaho National Engineering Laboratory (INEL), which is currently referred to as the Controlled Aspiration Process (CAP). The CAP process is set forth in detail in U.S. Pat. No. 4,919,853 issued to Alvarez and Watson on Apr. 24, 1990, and entitled "Apparatus And Method For Spraying Liquid Materials", the disclosure of which is herein incorporated by reference. The CAP process of spray-forming metals aspirates a molten metal into the throat of a converging/diverging gas nozzle, where the liquid is atomized into a directed spray of rapidly cooling droplets. The gas flowing in the nozzle may be ambient air or an inert gas which then accelerates the aspirated molten metal droplets toward a suitable substrate, against which the droplets impact before completely solidifying. Under ideal operating conditions, the incident metal consolidates into a suitable deposit.
Some problems occur with reproducible ideal operating conditions. In some instances, the molten metal does not atomize into a uniform cross-section spray. Aspiration only works within a narrow range of gas supply pressures. This difficulty is heightened by liquids within certain properties, such as, for example, kinematic high viscosity. Aspiration also limits the location of the liquid feed tube within the throat area of the nozzle. Aspiration limits particle size, particle size distributions, particle velocities, particle cooling rates, nozzle geometry, etc. Accordingly, it would be desirable to have an alternative means and method for atomizing the molten metal within a spray nozzle, as to provide for greater flexibility for controlling the properties of the spray which in turn dictate the properties of the spray-formed deposit.